In a 30-year-old patient with no known comorbidities who sustained polytrauma from a road traffic accident with a subarachnoid hemorrhage and is receiving mannitol, with capillary blood glucose 250 mg/dL, urine output 250 mL/h, serum sodium 149 mmol/L, potassium 5.5 mmol/L, and chloride 124 mmol/L, what is the likely cause of these electrolyte abnormalities and what treatment should be given?

Mannitol-Induced Hypernatremia with Hyperkalemia and High Urine Output

This patient is experiencing mannitol-induced osmotic diuresis with resulting hypernatremia, hyperkalemia, and hyperchloremia—stop the mannitol immediately and replace urinary losses with hypotonic fluids while closely monitoring electrolytes.

Underlying Pathophysiology

The clinical picture is classic for mannitol toxicity in the setting of traumatic brain injury with SAH 1, 2. The constellation of findings indicates:

• Hypernatremia (149 mmol/L): Mannitol causes obligatory osmotic diuresis with loss of water in excess of electrolytes, leading to hypernatremia 2
• Hyperkalemia (5.5 mmol/L): In polytrauma with crush injury components, potassium is released from damaged tissue; mannitol-induced dehydration concentrates serum potassium 3
• Hyperchloremia (124 mmol/L): Excessive normal saline administration combined with mannitol's effects 4
• High urine output (250 mL/hr): Mannitol's osmotic diuretic effect 2
• Hyperglycemia (250 mg/dL): Stress response from polytrauma

Immediate Management Algorithm

1. Stop Mannitol Immediately

The FDA label explicitly warns that mannitol causes hypernatremia through excessive water loss and can precipitate acute kidney injury 2. With sodium already at 149 mmol/L and high urine output, continued mannitol will worsen hypernatremia and potentially cause irreversible renal failure 2.

2. Address Hyperkalemia (5.5 mmol/L)

While this is mild hyperkalemia, in polytrauma it may worsen rapidly:

• Avoid potassium-containing fluids (Lactated Ringer's, Hartmann's solution) as these are contraindicated in crush injury/polytrauma 3
• Monitor ECG for peaked T-waves
• If K+ rises >6.0 mmol/L, initiate standard hyperkalemia treatment (calcium gluconate for cardiac protection, insulin-dextrose, consider dialysis if refractory) 5

3. Fluid Replacement Strategy

For hypernatremia correction:

• Use 0.45% saline (half-normal saline) to replace ongoing urinary losses 6
• Target correction rate: ≤10 mmol/L per 24 hours to avoid osmotic demyelination syndrome 7, 8
• Calculate free water deficit: 0.6 × body weight (kg) × [(current Na/140) - 1]
• Replace urinary losses mL-for-mL with hypotonic fluid

Critical caveat: In SAH patients, fluid restriction is contraindicated due to vasospasm risk 7. However, this patient needs controlled hypotonic fluid replacement, not restriction.

4. Monitoring Requirements

Check every 4-6 hours initially:

• Serum sodium, potassium, chloride
• Serum osmolality and calculate osmolar gap (to detect residual mannitol) 9
• Urine output
• Blood glucose
• Renal function (creatinine, BUN)

If osmolar gap remains elevated, consider hemodialysis for mannitol removal, monitoring corrected sodium (tonicity) rather than measured sodium to avoid overcorrection 9.

5. Alternative Osmotherapy if Needed

If intracranial pressure control is still required:

• Switch to 3% hypertonic saline in small boluses (1 mL/kg over 15-20 minutes) 1, 10
• Hypertonic saline is equally effective as mannitol for ICP control in SAH but causes less osmotic diuresis 1, 10
• Target serum sodium 145-155 mmol/L (therapeutic hypernatremia for brain protection)
• Monitor serum sodium every 4 hours

6. Hyperglycemia Management

Target glucose 140-180 mg/dL with insulin infusion 6, 11. Avoid hypoglycemia as this worsens neurological outcomes in SAH 11.

Key Pitfalls to Avoid

1. Do not continue mannitol with sodium >145 mmol/L—this risks irreversible renal failure 2
2. Do not use isotonic saline for maintenance—this will worsen hypernatremia 12, 4
3. Do not correct sodium too rapidly (>10 mmol/L/day)—risk of osmotic demyelination 7, 8
4. Do not restrict fluids in SAH patients—increases vasospasm risk 7
5. Do not give potassium-containing solutions in polytrauma 3
6. Do not use 4% albumin in severe TBI—associated with increased mortality 1

Specific Treatment Orders

1. Discontinue mannitol
2. Start 0.45% saline at 150-200 mL/hr (matching urine output)
3. Add 20 mEq KCl per liter only if K+ drops <4.0 mmol/L
4. Insulin infusion targeting glucose 140-180 mg/dL
5. Foley catheter for strict I/O monitoring
6. Electrolytes every 4 hours × 24 hours, then every 6 hours
7. If ICP control needed: 3% NaCl 1 mL/kg bolus over 15-20 minutes PRN

This approach prioritizes preventing further renal injury from mannitol while safely correcting the hypernatremia and managing the polytrauma complications 1, 2, 9.